The present invention relates to cutting tools and, in particular, to indexable cutting inserts. More particularly, the present invention relates to those positive rake inserts which may be held in a toolholder designed to accept a negative rake insert, and which are invertable as well as indexable.
Positive rake inserts require less power and produce a better finish than negative rake inserts in most applications. They typically required a toolholder specifically designed to hold a positive rake insert and were not invertable. Negative/positive, or N/P, inserts were then developed. These N/P inserts, while retaining a positive rake surface, can be held in a toolholder designed to take negative rake inserts and can also be invertable. Examples of N/P insert designs are shown in U.S. Pat. Nos. 3,399,442 and 3,557,416, both assigned to applicant corporation.
In inserts having a positive rake surface, the end clearance face (or peripheral wall) and rake surface form an acute angle at the cutting edge. This structure produces an inherently weaker cutting edge than that found in negative rake inserts in which this included angle is 90 degrees or more. The cutting edges of positive rake and N/P inserts are, therefore, subject to a greater degree of crumbling than encountered in negative rake inserts.
In those N/P inserts which are invertable, the cutting edge and positive rake surface directly below the cutting edge doing the cutting are typically not in abutment with the anvil of the toolholder seat in order to avoid damage to the lower cutting edge. This unsupported portion of the insert, especially in the corner or nose area, is thereby subjected to a bending moment which can lead to early failure of the nose area.